This invention relates to the processing of analogue and digital video signals and is particularly concerned with restoration of picture sharpness, ie. with recovery of wanted high frequency signal components lost or reduced in the process of video recording or transmission. Traditionally, such processing is known as aperture correction or edge enhancement.
A well known enhancement technique is to determine the second differential of the signal, multiply it by a coefficient (which determines the degree of correction) and add the result to the input signal. The frequency response of such an enhancer is linked to its pulse response by the well-known Fourier transform. A fundamental limitation of this kind of enhancer is that it is impossible to boost the high frequency components without visible and annoying overshoots in the pulse and step responses.
There is another known, non-linear edge enhancement technique based on the synthesis of high frequency components by extension of the low-frequency part of the signal spectrum. This approach includes the calculation of the first and second derivatives of an input signal, then multiplication of said first derivative by the clipped second derivative and, finally, summation of the resulting correction signal with the incoming signal. Such processing provides improved pulse response, although the pulse response is not completely free from unwanted overshoots or ripples, and periodic picture textures are degraded.